1. Technical Field
The present disclosure relates in general to wireless communication devices and in particular to power amplifiers in wireless communication devices.
2. Description of the Related Art
Envelope Tracking (ET) power amplifiers deliver higher efficiency at equal spectral performance to conventional power amplifiers. The efficiency improvement is achieved by dynamically changing the power amplifier supply voltage in response to the instantaneous changes in amplitude of the radio frequency (RF) signal to be amplified. This is in contrast to a simpler, but less effective efficiency enhancement called Average Power Tracking (APT), in which the power amplifier supply voltage is varied in response to the average amplitude of the RF signal to be amplified.
Another general characteristic of ET power amplifiers is that their efficiency tends to fall as the power output falls. This is because the ET supply voltage does not track the RF amplitude below a certain threshold voltage. At low supply voltages the power amplifier (PA) gain and phase tend to vary more rapidly, and it becomes difficult to control the PA power output or maintain acceptable modulation accuracy and spectral performance. For this reason, the ET system is designed so that the power amplifier supply voltage closely follows the RF signal amplitude at high instantaneous amplitudes, but at low instantaneous amplitudes the voltage remains at a fixed threshold. As the average power falls, the instantaneous power and supply voltage are below the threshold a greater percentage of the time, the voltage is at a fixed value a greater percentage of the time, and the supply system begins to look increasingly like the APT system.
Some conventional systems have used these efficiency characteristics to provide transitions between using the ET system at higher average powers and using the APT system at lower average powers in order to maintain optimal efficiency over all power levels. However, a transition between ET and APT techniques can cause undesired transients affecting signal quality or spectrum.